(i) Field of the Invention
The present invention relates to a laser beam-recording medium comprising a substrate, a light-absorbing layer and a heat-sensitive color-developing layer thereon and having a reflectance of 15to 65% at a laser beam dominant wave length on the surface thereof, a method for recording by irradiating the recording medium with a laser beam, and a device used in this recording method.
(ii) Description of the Prior Art
In a direct recording system which does not require any development and fixing, heat-sensitive recording papers in which a basic dye and an organic developer are used as color developing agent are excellent in operation and maintenance. For this reason, the direct recording system is widely utilized in facsimiles, printers and the like.
In this system, however, the recording is thermally made by bringing a thermal head or an exothermic IC pen into direct contact with a heat-sensitive recording paper, and therefore melted colored substances adhere to the thermal head or the exothermic IC pen, so that troubles such as dregs adhesion and sticking take place, which brings about record obstruction and impairs record quality inconveniently.
In particular, when a line is depicted continuously in a recording direction as in the case of a plotter printer, it is impossible to avoid the trouble of the dregs adhesion.
Furthermore, when the recording is made by the thermal head, it is difficult to heighten an image resolution up to 8 dots/mm or more.
Thus, as techniques by which troubles such as the dregs adhesion and the sticking are solved and by which the resolution is more improved, some non-contact recording systems using light have been suggested.
Japanese Patent Laid-open Publication No. 4142/1979 discloses a heat-sensitive recording medium prepared by coating a substrate with a heat-sensitive recording layer mainly comprising a leuco dye, and in this recording medium, a metallic compound having a lattice defect is used. This metallic compound absorbs beams in visible and infrared regions, so that thermal conversion occurs, thereby achieving the heat-sensitive recording.
Japanese Patent Laid-open Publication No. 209594/1983 discloses an optical recording medium prepared by laminating at least one set of a near infrared absorbent layer having an absorption wave length in a near infrared region of 0.8 to 2 .mu.m and a heat-sensitive color-developing material layer onto a substrate, and Japanese Patent Laid-open Publication No. 94494/1983 discloses a recording medium prepared by superposing, on a base material, a layer containing one or more kinds of heat-sensitive color-developing materials and a layer containing one or more kinds of near infrared absorbents comprising compounds having a maximum absorption wave length in near infrared rays of 0.7 to 3 .mu.m. The recording onto these recording mediums can be carried out by the use of a thermal plate and a laser beam having a wave length in the vicinity of near infrared rays.
Judging from the fact that the optical recording devices are prevalently used in office works and usual measurements, the output of the usable lasers must be low. In small-sized lasers which can be used in such recording devices, a dominant wave length is from 650 to 1500 .mu.m in the vicinity of near infrared rays.
Furthermore, in Japanese Patent Laid-open Publication No. 209594/1983 and the like mentioned above, the near infrared absorbent and the heat-sensitive color-developing material are applied on the substrate or the base material. That is, these materials are mixed and the resulting mixture is then applied onto the substrate or the base material, or alternatively when the heat-sensitive color-developing material is first applied on the substrate or the base material, and the near infrared absorbent is then applied on this material layer.
When an optical recording medium comprising the combination of the conventional heat-sensitive material and a light absorbent is irradiated with near infrared rays, recording is carried out on the so-called heat-sensitive color-developing dye material comprising a basic color dye and an organic developer which are used in the field of heat-sensitive recording papers. Therefore, the recorded color information fades and vanishes inconveniently, when brought into contact with a solvent, a plasticizer, an oil or a fat.
In the field of the heat-sensitive recording papers, it is known to form a protective layer of a water-soluble binder on the heat-sensitive recording layer so as to improve the above-mentioned inconvenient point, but the protective layer coating film is thin, and its barrier properties are imperfect. After all, the fading and disappearance of the recorded color information by the solvent or the like cannot be prevented completely. It can also be contrived to increase the thickness of the protective coating film. However, the heat energy from a thermal head lowers in inverse proportion to the square of the thickness of the protective layer, and thus, if the thickness of the protective layer is 4 microns or more, it is difficult to perform the recording dynamically. In addition, thermal diffusion increases simultaneously, so that recording density and resolution deteriorate. Moreover, the protective coating film is hard to dry in the manufacturing process of the recording papers, and hence workability is also bad.
The above-mentioned publications disclose the optical recording mediums in which non-hydrophilic materials such as plastic plates and films are used as the base materials, and the optical recording mediums in which papers which are hydrophilic materials are used as the base materials. However, they do not elucidate conditions which the practical optical recording mediums have in connection with a laser beam.
In the case of photodiscs, reading can be mechanically made, and so an optical contrast in a reading wave length is enough. However, in the recording medium of the present invention, such a contrast as to be readable by the naked eye is required, since the recorded information is read directly visually by a man.
In particular, the substrate of the optical recording medium which is a hydrophilic material such as a paper is much rougher on the surface thereof than the photodisc in which a non-hydrophilic material such as a plass and a plastic plate is used as the base material. In the case that the substrate is the paper, the depth of each recess on the rough surface of this paper is usually from 5 to 6 microns, and the winding of the paper is also great at times. Therefore, with regard to the stop down of a focal point, some allowance is inevitably necessary.